Carriers such as electrons and holes inside the Brillouin zone of complex semiconducting materials can form bound states (excitons, biexcitons, etc.). For obtaining the corresponding eigenstates (e.g., through the Wannier or Bethe-Salpeter equation) and dynamics (e.g., cluster expansion) the number of involved electrons and holes as well as the accuracy is limited by the appearing high-dimensional tensors (i.e., wave functions or correlations). These tensors can be efficiently represented and manipulated via tensor network methods. We show how tensor networks formulated via classic logic gates can be used to treat electron-hole complexes inside the Brillouin zone. The method is illustrated for the exciton and biexciton states of a single-layer transition-metal dichalcogenide ${\mathrm{MoS}}_2$-like model system.

Breaking of the inversion symmetry at the interface between different materials may dramatically enhance spin-orbit interaction in the vicinity of the interface. We incorporate the effects of this interfacial spin-orbit coupling (ISOC) into the standard drift-diffusion theory by deriving generalized boundary conditions for diffusion equations. Our theoretical scheme is based on symmetry arguments, providing a natural classification and parametrization of all spin-charge and spin-spin conversion effects that occur due to ISOC at macroscopically isotropic interfaces between nonmagnetic materials. We illustrate our approach with specific examples of spin-charge conversion in hybrid structures. In particular, for a lateral metal-insulator structure we predict an “ISOC-gating” effect which can be used to detect spin currents in metallic films with weak bulk SOC.

We report on the observation of the strong-coupling regime between quantum well excitons and a high incidence “Brewster cavity mode” previously identified as the generalized Brewster angle condition in multilayer structures [H. F. Mahlein, J. Opt. Soc. Am. 64, 647 (1974)]. This propagating mode is inside the light cone and therefore can be accessed from the top side of the sample without the need for prism or grating coupling methods. The observed anticrossing is clear evidence of the strong light-matter coupling regime. All the results are accurately reproduced by transfer matrix simulations. These results demonstrate the high potential of such structures for the study of propagating polaritons at high $k$, which could be harnessed for the realization of polaritonic circuit devices.

A pair of anisotropic exceptional points (EPs) of arbitrary order is found in a class of non-Hermitian random systems with asymmetric hoppings. Both eigenvalues and eigenvectors exhibit distinct behaviors when these anisotropic EPs are approached from two orthogonal directions in the parameter space. For an order-$N$ anisotropic EP, the critical exponents $\nu$ of phase rigidity are $\left(N-1\right)/2$ and $N-1$, respectively. These exponents are universal within the class. The order-$N$ anisotropic EPs split and trace out multiple ellipses of EPs of order 2 in the parameter space. For some particular configurations, all the EP ellipses coalesce and form a ring of EPs of order $N$. Crossover to the conventional order-$N$ EPs with $\nu =\left(N-1\right)/N$ is discussed.

Topological order, the hallmark of fractional quantum Hall states, is primarily defined in terms of ground-state degeneracy on higher-genus manifolds, e.g., the torus. We investigate analytically and numerically the smooth crossover between this topological regime and the Tao-Thouless thin torus quasi-one-dimensional (1D) limit. Using the wire-construction approach, we analyze an emergent charge density wave (CDW) signifying the breakdown of topological order, and relate its phase shifts to Wilson loop operators. The CDW amplitude decreases exponentially with the torus circumference once it exceeds the transverse correlation length controllable by the interwire coupling. By means of numerical simulations based on the matrix product states (MPS) formalism, we explore the extreme quasi-1D limit in a two-leg flux ladder and present a simple recipe for probing fractional charge excitations in the $\nu =1/2$ Laughlin-like state of hard-core bosons. We discuss the possibility of realizing this construction in cold-atom experiments. We also address the implications of our findings to the possibility of producing non-Abelian zero modes. As known from rigorous no-go theorems, topological protection for exotic zero modes such as parafermions cannot exist in 1D fermionic systems and the associated degeneracy cannot be robust. Our theory of the 1D–2D crossover allows us to calculate the splitting of the degeneracy, which vanishes exponentially with the number of wires, similarly to the CDW amplitude.

We theoretically revealed that a weak photoexcitation achieves the electric polarization-inversion with approximately $18\%$ of all the charges, which was interpreted as a superimposition of multiexciton states, from the charge-ordered ferroelectric ground state of ${\left(\mathrm{TMTTF}\right)}_2{\mathrm{PF}}_6$ at absolute zero temperature. Regarding a relative change of electric polarization ($\mathrm{\Delta }P/P$), the photoexcitation corresponds to $36\%$, which is much larger than $\mathrm{\Delta }P/P$ of other typical organic materials. The photoexcitation of $\mathrm{\Delta }P/P\sim 36\%$ corresponds to the single peak of the optical conductivity in the low-energy region, which was also observed at 10 K. Therefore, the value of $\mathrm{\Delta }P/P\sim 36\%$ can be achieved in the early stage of the ultrafast photoinduced dynamics of the material. This fact is useful not only for applications of this material and other analogous materials in optical devices, but also for research toward controlling electric polarizations by light, which is one of the recent attracting issues in photoinduced phase transition phenomena. Theoretical calculations are based on a quarter-filled one-dimensional effective model with appropriate parameters and 50 unit cells.

We investigate the influence of Auger electrons on the efficiency of optical spin pumping of donor bound electrons in isotopically enriched silicon by means of time-resolved pump-probe absorption spectroscopy. The experimentally observed drastic shortening of the spin relaxation time with increasing optical excitation results from inelastic scattering between free and localized electrons and an interconnected Orbach-type spin relaxation process. The maximal steady-state degree of polarization of the localized electron spins and the probe-beam-induced depolarization dynamics reveal that the fast, nonthermal energy dissipation of the conduction band electron distribution is detrimental for efficient optical spin initialization. In fact, each absorbed photon creates an Auger electron with enough energy to depolarize the spin of $\approx 30$ donor bound electrons but the fast, nonthermal energy dissipation by phonons reduces this spin relaxation efficiency to approximately unity.

We investigate the degree of entanglement quantified by the concurrence of photon pairs that are simultaneously emitted in the biexciton-exciton cascade from a quantum dot in a cavity. Four dot-cavity configurations are compared that differ with respect to the detuning between the cavity modes and the quantum dot transitions, corresponding to different relative weights of direct two-photon and sequential single-photon processes. The dependence of the entanglement on the exciton fine-structure splitting $\delta$ is found to be significantly different for each of the four configurations. For a finite splitting and low temperatures, the highest entanglement is found when the cavity modes are in resonance with the two-photon transition between the biexciton and the ground state and, in addition, the biexciton has a finite binding energy of a few meV. However, this widely used configuration is rather strongly affected by phonons such that other dot-cavity configurations, that are commonly regarded as less suited for obtaining high degrees of entanglement, become more favorable already at temperatures on the order of $10\mathrm{K}$ and above. If the cavity modes are kept in resonance with one of the exciton-to-ground-state transitions and the biexciton binding energy is finite, the entanglement drastically drops for positive $\delta$ with rising temperatures when $T$ is below $\simeq 4\mathrm{K}$, but is virtually independent of the temperature for higher $T$.

We report a theoretical study of the impact of dipolar interactions on the magnetic phases and thermal hysteresis of superparamagnetic Gd nanoparticle clusters. We consider Gd nanoparticles with size ranging from 5.5 to 10 nm, arranged with uniform density in a few-hundred-nanometer-sized ellipsoidal clusters. We show that in densely packed systems the dipolar interaction may grant thermal stabilization of the individual nanoparticle moments at high-temperature values, leading to large thermal loop widths, tunable by external magnetic fields of moderate strength. We have found, for instance, that with an external magnetic field strength of 85 Oe one may reach thermal loop widths ranging from 88 K for 5.5-nm-diam Gd nanoparticle clusters to 660 K for 8-nm-diam Gd nanoparticle clusters.

We present AC and DC magnetometry, heat capacity, muon spin relaxation ($\mu \mathrm{SR}$), and resonant inelastic x-ray scattering (RIXS) studies of the pyrochlore osmate ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. Consistent with previous results, we observe a small Curie-Weiss effective moment of approximately 0.4 ${\mu }_{\mathrm{B}}$ and spin freezing at a temperature $T_f\simeq 5\mathrm{K}$. On the other hand, our analysis of the RIXS spectra with single-ion energy-level calculations implies a nonmagnetic $J_{\mathrm{eff}}=0$ ground state on the ${\mathrm{Os}}^{4+}$ sites, with the spin-orbit interaction, Hund's coupling, and trigonal distortion of ${\mathrm{OsO}}_6$ octahedra, all important in modeling the observed spectra. From high-field magnetization data, we show that the paramagnetic moment is most likely due to large moments ${\mu }_{\mathrm{eff}}\simeq 3{\mu }_{\mathrm{B}}$ on only a small fraction $f\simeq 0.02$ of Os sites. We suggest disorder-related effects, such as oxygen nonstoichiometry or site interchange between Os and Y ions, as the most likely explanation for the magnetic response of this material.

We consider a metal with interaction mediated by fluctuations of an order parameter, which condenses at a quantum critical point (QCP). This interaction gives rise to fermionic incoherence in the normal state and also mediates pairing. Away from a QCP, the pairing restores fermionic coherence almost immediately below $T_c$. We show that near a QCP, fermions regain coherence only below a certain $T_{\text{cross}}$, which is smaller than the onset temperature for the pairing $T_p$. At $T<T_{\text{cross}}$ the system behavior is conventional in the sense that both the density of states (DOS) and the spectral function (SF) have sharp gaps, which close in as $T$ increases. At higher $T_{\text{cross}}<T<T_p$, the DOS has a dip, which fills in with increasing $T$, while the SF shows either the same behavior as the DOS, or has a peak at $\omega =0$, depending on the position on the Fermi surface, leading to a Fermi arc. We argue that phase fluctuations are strong at $T>T_{\text{cross}}$, and the actual $T_c\ge T_{\text{cross}}$, while at larger $T_c<T<T_p$ the system displays a pseudogap behavior. We argue that our theory explains the crossover from gap closing to gap filling, observed in cuprate superconductors at $T\le T_c$, and the persistence of the dip in the DOS and and the Fermi arc above $T_c$.

Long-range interactions are relevant for a wide range of phenomena in physics where they often present a challenge to theory. In condensed matter, the interplay of Coulomb interaction and disorder remains largely an unsolved problem. In two-dimensional films the long-range part of the Coulomb interaction may be screened by a nearby metallic overlay. This technique is employed in this work to present experimental evidence of its effectiveness in limiting the spatial range of the Coulomb interaction. We use this approach to study the effects of the long-range Coulomb interaction on the out-of-equilibrium dynamics of electron glasses using amorphous indium oxide films. The results demonstrate that electronic relaxation times, extending over thousands of seconds, do not hinge on the long-range Coulomb interaction or on the presence of a real gap in the density of states. Rather, they emphasize the dominant role played by disorder in controlling the slow thermalization processes of Anderson insulators taken far from equilibrium.

We investigate the typicality of the growth behavior of the out-of-time-ordered commutator (OTOC) in the many-body localized (MBL) quantum spin chains across random disorder realizations. In the MBL phase of the Heisenberg XXZ chain, we find that the estimate of the OTOC fluctuates significantly with the disorder realizations at the intermediate times of the main growth. Despite the consequent failure of the disorder average in the MBL phase, we argue that the characteristic behavior of the OTOC growth can still be identified by going through individual disorder realizations. We find that a power-law-type growth behavior appears typically after a disorder-dependent relaxation period, which is very close to the $t^2$ form derived in the effective Hamiltonian of a fully MBL system. The characteristic growth behavior observed at an individual disorder realization is robust in our tests with various state preparations and also verified in another MBL system of the random-transverse-field quantum Ising chain in a uniform longitudinal field.

Modulating electronic structure of two-dimensional materials represents an exciting avenue for tailoring their optoelectronic properties. Here, we identify a strain-induced, cooperative effect of intraband and interband excitations contributing to high-harmonic generation (HHG) in prototype dichalcogenide ${\mathrm{MoS}}_2$ monolayer. We find that besides the dominant intraband contributions, interband current is also indispensable in modulating HHG. The HHG yields increase linearly with the compressive strain since flatter band dispersion and Berry curvature enhance both interband and intraband dynamics. Band structure can be retrieved with high reliability by monitoring the strain-induced evolution of HHG spectra, suggesting that strain not only provides an additional knob to control HHG in solids, but also marks a way towards a complete understanding of underlying microscopic mechanisms.

We study the acoustic response of flat-meniscus bubbles trapped in the grooves of a microstructured hydrophobic substrate immersed in water. In the first part of the paper, we consider a single bubble subjected to a normally incident plane wave. We use the method of matched asymptotic expansions, based on the smallness of the gas-to-liquid density ratio, to describe the near field of the groove, where the compressibility of the liquid can be neglected, and an acoustic region, on the scale of the wavelength, which is much larger than the groove opening in the resonance regime of interest. We find that bubbles trapped in grooves support multiple subwavelength resonances, which are damped—radiatively—even in the absence of dissipation. Beyond the fundamental resonance, at which the pinned meniscus is approximately parabolic, we find a sequence of higher-order antiresonance and resonance pairs; at the antiresonances (whose frequencies are independent of the gas properties and groove size), the gas is idle and the scattering vanishes, while the liquid pressure is in balance with capillary forces.

In the second part of the paper, we develop a multiple-scattering theory for dilute arrays of trapped bubbles, where the frequency response of a single bubble enters via a scattering coefficient. For an infinite array and subwavelength spacing between the bubbles, the resonances are suppressed by an interference effect associated with the strong logarithmic interactions between quasistatic line sources; the antiresonances are robust. In contrast, for finite arrays, however large, we find strong and highly oscillatory deviations from the frequency response of an infinite array in a sequence of intervals about the resonance frequencies of a single bubble; these deviations are shown to be associated with edge excitation, in the finite case, of surface “spoof plasmon” waves, which exist in the infinite case precisely in the said frequency intervals; the resonant peaks in these intervals correspond to the formation of standing surface waves in the finite array.

We study the accuracy of analytical wave-function-based many-body methods derived by energy minimization of a Jastrow-Feenberg ansatz for electrons (the “Fermi hypernetted chain/Euler Lagrange” approach). Approximations to avoid the complexity of the fermion problem are chosen to parallel successful boson theories and to be computationally efficient. For the three-dimensional homogeneous electron gas, we calculate the correlation energy, the pair distribution function, and the static structure function in comparison with simulation results. We also present a variant of theory which is interpreted as an approximate, self-consistent sum of ladder and ring diagrams of perturbation theory. The theory performs particularly well in the highly dilute density regime.

We report on the importance of resonant-tunneling processes on quantum transport through interacting quantum-dot spin valves. To include Coulomb interaction in the calculation of the tunneling magnetoresistance (TMR), we reformulate and generalize the recently developed, numerically exact method of iterative summation of path integrals (ISPI) to account for spin-dependent tunneling. The ISPI scheme allows us to investigate weak to intermediate Coulomb interaction in a wide range of gate and bias voltage and down to temperatures at which a perturbative treatment of tunneling severely fails.

We show the explicit connection between two distinct and complementary approaches to the fractional quantum Hall system (FQHS): the quantum wires formalism and the topological low-energy effective description given in terms of an Abelian Chern-Simons theory. The quantum wires approach provides a description of the FQHS directly in terms of fermions arranged in an array of one-dimensional coupled wires. In this sense it is usually referred to as a microscopic description. On the other hand, the effective theory has no connection with the microscopic modes, involving only the emergent topological degrees of freedom embodied in an Abelian Chern-Simons gauge field, which somehow encodes the collective dynamics of the strongly correlated electrons. The basic strategy pursued in this work is to bosonize the quantum wires system and then consider the continuum limit. By examining the algebra of the bosonic operators in the Hamiltonian, we are able to identify the bosonized microscopic fields with the components of the field strength (electric and magnetic fields) of the emergent gauge field. Thus our study provides a bridge between the microscopic physical degrees of freedom and the emergent topological ones without relying on the bulk-edge correspondence.